Imaging lens system

ABSTRACT

Provided is an imaging lens system which can sufficiently meet the demands for further reduction of size and weight and for further improvement of the optical performance, and is also capable of improving the productivity. The imaging lens system comprises, from the object side, a diaphragm, a first lens as a positive meniscus lens whose convex surface facing the object side, and a second lens as a positive lens whose convex surface facing the image surface side, wherein following expressions are to be satisfied 1.25≧L/fl≧0.8, 1≧f 1 /f 2 ≧0.55, 1.8≧f 1 /fl≧1, 4≧f 2 /fl≧1.5, 1≧d 2 /d 1 ≧0.5, 0.27 ≧d 1 /fl≧0.1, 0.27≧d 3 /fl≧0.1 (where, L: entire length of the lens system, fl: focal distance of entire lens system, f 1 : focal distance of the first lens, f 2 : focal distance of the second lens, d 1 : center thickness of the first lens, d 2 : distance between the first lens and second lens on the optical axis, d 3 : center thickness of the second lens).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an imaging lens system and,particularly, to an imaging system of two-lens structure which iscapable of reducing the size and weight and improving the opticalperformance and productivity. The imaging system is used for an imagepickup device that forms images of objects such as scenery and humanfigures on an image taking surface of a solid image sensor element suchas a CCD, CMOS, or the like, which is mounted on a portable computer, atelevision telephone, a cellular phone, and the like.

2. Description of the Related Art

Recently, thee has been a remarkably increasing demand for a camerawhich utilizes a solid image sensor element such as a CCD, CMOS, or thelike, which is to be mounted on a portable telephone, a portablecomputer, and a television telephone, for example. Such camera isnecessary to be mounted on a limited mount space so that it is desiredto be small and light.

Accordingly, an imaging lens system used for such camera is alsorequired to be small and light. Thus, conventionally, a single-structurelens system using a single lens is used for such imaging lens system.

Such lens system with single-lens structure is sufficient when appliedto a solid image sensor element referred to as CIF which has resolutionof about 110,000 pixels. However, recently, use of a solid image sensorelement referred to as VGA, which has higher resolution of about 300,000pixels, has been investigated. In order to fully use the ability of theresolution of such solid image sensor element with the high resolution,the conventional lens system with the single-lens system is notsufficient.

Thus, conventionally, various types of lens systems with two-lensstructure and three-lens structure have been proposed, which are moreexcellent in optical performance compared to that of the lens systemwith a single-lens structure.

In those cases, the three-lens-structure lens system enables toeffectively correct each aberration that causes deterioration of theoptical performance, thereby achieving extremely high opticalperformance. However, there are a large number of components required inthe three-lens-structure lens system so that reduction of the size andweight is difficult, and manufacturing cost increases since highprecision is required in each component.

On the contrary, the two-lens-structure lens system exhibits higheroptical performance than the single-lens-structure lens system althoughit is not as high as that of the three-lens-structure lens system. Thus,it is considered a preferable lens system for a small andhigh-resolution solid image sensor element.

Conventionally, as such two-lens-structure lens system, a large numberof so-called retro-focus type lens systems in which a negative lens anda positive lens are combined have been proposed. However, although suchretro-focus type lens system can reduce the cost by decreasing thenumber of components, reduction of the size and weight close to that ofthe single-lens-structure lens system is practically not possible due toits structure where the back focus distance becomes long.

Further, as another two-lens-structure lens system, there is so-called atelephoto-type lens system in which a positive lens and a negative lensare combined. However, such telephoto-type lens system is originallydeveloped for film photos so that the back focus distance is too short.Also, there is an issue of telecentricity so that it is difficult to useit as it is for an imaging lens system for a solid image sensor element.

Further, conventionally, the main stream structure of thetwo-lens-structure lens system or the three-lens-structure lens systemhas been formed to have a diaphragm disposed between two lenses whichare adjacent to each other in the optical axis direction (for example,see Japanese Patent Unexamined Publication No. 2004-163850 and JapanesePatent Unexamined Publication No. 170460).

However, recently, there has been more increasing demand for stillfurther improvement of the optical performance in this type of imaginglens system in addition to the reduction of size and weight. Thus, withthe structure in which a diaphragm is disposed between two lenses as theimaging lens system disclosed in Japanese Patent Unexamined PublicationNo: 2004-163850 and Japanese Patent Unexamined Publication No. 170460,it is difficult to achieve both the reduction of size and weight andfurther improvement of the optical performance. Furthermore, it isdifficult to align with the characteristic of a sensor (incident anglefor the sensor).

SUMMARY OF THE INVENTION

The present invention has been designed to overcome the aforementionedproblems. It is therefore an object of the present invention to providean imaging lens system which can fully meet the demand for reduction ofthe size and weight and further improvement of the optical performanceand also can improve the productivity.

In this specification, “productivity” is not only the productivity inthe case of mass-producing the imaging lens systems (for example,moldability, cost, and the like when mass-producing the imaging lenssystems by injection molding) but also includes simplicity ofprocessing, manufacturing, and the like of equipment which is used formanufacturing the imaging lens systems (for example, simplicity and thelike of processing a die which is used for the injection molding).

In order to achieve the foregoing object, an imaging lens systemaccording to a first aspect of the present invention is an imaging lenssystem used for forming an image of an object on an image taking surfaceof a solid image sensor element. The imaging lens system comprises:

in order from an object side towards an image surface side, a diaphragm,a first lens which is a meniscus lens having positive power whose convexsurface facing the object side, and a second lens which is a lens havinga positive power whose convex surface facing the image surface, wherein

conditions expressed by each of following expressions (1)-(7) are to besatisfied;1.25 ≧L/fl≧0.8   (1)1≧f ₁ /f ₂>0.55   (2)1.8≧f ₁ /fl≧1   (3)4≧f ₂ /fl≧1.5   (4)1≧d ₂ /d ₁≧0.5   (5)0.27≧d ₁ /fl≧0.1   (6)0.27≧d ₃ /fl≧0.1   (7)where, L: entire length of the lens system

fl: focal distance of entire lens system

f₁: focal distance of the first lens

f₂: focal distance of the second lens

d₁: center thickness of the first lens

d₂: distance between the first lens and the second lens on the opticalaxis

d₃: center thickness of the second lens

In the present invention according to the first aspect, the diaphragm isdisposed at a point closest to the object side. Thus, it enables tomaintain high telecentricity so that the incident angle of the lightrays with respect to the sensor of the solid image sensor element can bemodified.

In the present invention, disposing the diaphragm at a position closestto object side does not hinder the part of the object-side surface(convex surface) of the first lens, which is near the optical axis, frombeing disposed at a position closer to the object side than thediaphragm through the diaphragm. Even so, physically, the diaphragm isdisposed at a position closer to the object side than the first lens asa whole. Thus, it complies with what is disclosed in appended claims.

Further, in the present invention according to the first aspect, thefirst lens is formed as a meniscus lens having positive power whoseconvex surface facing the object side, the second lens is formed as alens having a positive power, and the power of each lens is designatedto satisfy the conditions expressed by each of the expressions (1)-(7).Therefore, it is possible to improve the productivity while achievingreduction of size and weight.

In the imaging lens system according to a second aspect, the second lensof the first aspect is formed as a meniscus lens.

With the present invention according to the second aspect, further, itbecomes possible to improve the optical performance of the peripherywithout imposing a burden on the shapes of the first lens and secondlens. Also, the light rays making incidence to the periphery of thesolid image sensor element can be more effectively utilized.

Further, in the imaging lens system according to a third aspect, anobject-side surface of the second lens of the first aspect is protrudedtowards the object side in the area near the optical axis and is alsoformed as an aspherical face with an inflection point.

With the present invention according to the third aspect, further, itenables to further improve the optical performance of the periphery byreducing a burden on the shapes of the first lens and second lens. Also,the light rays making incidence to the periphery of the solid imagesensor element can be more effectively utilized.

Furthermore, in the imaging lens system according to a fourth aspect, anouter end part of effective diameter in an object-side surface of thesecond lens according to the third aspect is positioned closer to theobject side than a point on the optical axis on the object-side surfaceof the second lens.

With the present invention according to the fourth aspect, the opticalperformance in the periphery can be more improved. Also, it is not onlyadvantageous at the time of handling the lens but also advantageous atthe time of assembling for making a unit by mounting the lens to abarrel.

Moreover, in the imaging lens system according to a fifth aspect, thediaphragm in any one of the first to fourth aspects satisfies afollowing expression;0.2≧S   (8)where, S: distance between the diaphragm on an optical axis and anoptical surface closest to the object side.

With the present invention according to the fifth aspect, further, itbecomes possible to more effectively maintain the telecentricity bysatisfying the expression (8). Thus, more reduction of the size andweight can be achieved.

Further, in the imaging lens system according to a sixth aspect, in anyof the first to fifth aspects, condition expressed by a followingexpression (9) is to be further satisfied;0.8≧Bfl/fl≧0.4   (9)where, Bfl: back focus distance (distance from a lens end surface to animage taking surface on an optical axis (air reduced length)).

With the present invention according to the sixth aspect, reduction ofthe size and weight can be more effectively achieved by satisfying theexpression (9). Also, the productivity can be more improved and it canbe more easily handled at the time of assembling.

Furthermore, in the imaging lens system according to a seventh aspect,in any of the first to sixth aspects, condition expressed by a followingexpression (10) is to be further satisfied;2.5≧Bfl≧1.2   (10)

With the present invention according to the seventh aspect, reduction ofthe size and weight can be more effectively achieved by satisfying theexpression (10). Also, the productivity can be more improved and it canbe more easily handled at the time of assembling.

Moreover, in the imaging lens system according to an eighth aspect, inany of the first to seventh aspects, condition expressed by a followingexpression (10) is to be further satisfied;−0.5≧r4/fl≧−6.0   (11)where, r₄: curvature radius of a surface of the second lens on the imagesurface side.

With the present invention according to the eighth aspect, it becomespossible to perform processing of the optical surface more easily bysatisfying the expression (11). Also, it enables to correct eachaberration in the periphery more excellently.

With the imaging lens system according to the first aspect of thepresent invention, it is possible to achieve an imaging lens systemwhich is small and light, excellent in optical performance, and also hasa good productivity.

Further, with the imaging lens system according to the second aspect ofthe present invention, in addition to the effect of the imaging lenssystem of the first aspect, it is possible achieve a small-size imaginglens system which can improve the optical performance while maintainingthe good productivity, and also is capable of effectively utilizing thelight rays that make incidence to the periphery of the solid imagesensor element.

Furthermore, with the imaging lens system according to the third aspectof the present invention, in addition to the effect of the imaging lenssystem of the first aspect, it is possible to exhibit more excellentoptical performance while maintaining the good productivity. Also, it ispossible to achieve a small-size imaging lens system which can moreeffectively utilize the light rays that make incident to the peripheryof the solid image sensor element.

Moreover, with the imaging lens system according to the fourth aspect ofthe present invention, in addition to the effect of the imaging lenssystem of the third aspect, it is possible achieve a small-size imaginglens system which is more excellent in the optical performance whileenabling to maintain the good productivity, and also is capable ofeffectively utilizing the light rays that make incidence to theperiphery of the solid image sensor element.

Further, with the imaging lens system according to the fifth aspect ofthe present invention, in addition to the effect of the imaging lenssystem in any one of the first to fourth aspects, further, thetelecentricty can be more effectively maintained. Also, it is possibleto achieve an imaging les system which is suitable for further reductionof size and weight.

Furthermore, with the imaging lens system according to the sixth aspectof the present invention, in addition to the effect of the imaging lenssystem in any one of the first to fifth aspects, further, it is possibleto achieve an imaging lens system which is more reduced in size andweight and has an excellent productivity.

Moreover, with the imaging lens system according to the seventh aspectof the present invention, in addition to the effect of the imaging lenssystem in any one of the first to sixth aspects, further, it is possibleto achieve an imaging lens system which is still more suitable forreducing size and weight and for improving the productivity.

Further, with the imaging lens system according to the eighth aspect ofthe present invention, in addition to the effect of the imaging lenssystem in any one of the first to seventh aspects, further, it ispossible to achieve an imaging lens system which is more excellent inthe optical performance and the productivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram for showing an embodiment of an imaginglens system according to the present invention;

FIG. 2 is a schematic diagram for showing FIRST EXAMPLE of the imaginglens system according to the present invention;

FIG. 3 show graphs for describing the spherical aberration, astigmatism,and distortion of the imaging lens system shown in FIG. 2;

FIG. 4 is a schematic diagram for showing SECOND EXAMPLE of the imaginglens system according to the present invention;

FIG. 5 show graphs for describing the spherical aberration, astigmatism,and distortion of the imaging lens system shown in FIG. 4;

FIG. 6 is a schematic diagram for showing THIRD EXAMPLE of the imaginglens system according to the present invention;

FIG. 7 show graphs for describing the spherical aberration, astigmatism,and distortion of the imaging lens system shown in FIG. 6;

FIG. 8 is a schematic diagram for showing FOURTH EXAMPLE of the imaginglens system according to the present invention;

FIG. 9 show graphs for describing the spherical aberration, astigmatism,and distortion of the imaging lens system shown in FIG. 8;

FIG. 10 is a schematic diagram for showing FIFTH EXAMPLE of the imaginglens system according to the present invention; and

FIG. 11 show graphs for describing the spherical aberration,astigmatism, and distortion of the imaging lens system shown in FIG. 10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of an imaging lens system according to the present inventionwill be described hereinafter by referring to FIG. 1.

As shown in FIG. 1, an imaging lens system 1 of the embodimentcomprises, in order from the object side towards the image surface side,a diaphragm 2, a resin-type first lens 3 which is a meniscus lens havinga positive power with its convex surface facing the object side, and aresin-type second lens 4 which is a lens having a positive power withits convex surface facing the image surface side.

In the followings, each of the lens surfaces of the first lens 3 and thesecond lens 4 on the object side is referred to as a first face thereof,and each lens surface on the image surface side is referred to as asecond face thereof, respectively.

On the second face side of the second lens 4, various filters 6 such asa cover glass, an IR cut filter, and a lowpass filter, and an imagesurface 7 which is a light-receiving surface of an image pickup elementsuch as a CCD or a CMOS are disposed, respectively. The various filters6 may be omitted as appropriate.

As the position of the diaphragm 2 gets closer to the image surfaceside, the position of exit pupil comes closer to the image surface side.Thus, it becomes difficult to maintain the telecentricity, and abaxiallight rays emitted from the imaging lens system 1 makes incidenceobliquely to the sensor of the solid image sensor element.

In the meantime, with the embodiment, it is possible to set the positionof the exit pupil far from the sensor surface (image taking surface) ofthe solid image sensor element by disposing the diaphragm 2 closest tothe object side.

Thereby, with the embodiment, it becomes possible to maintain the hightelecentricity, and the incident angle of the light rays against thesensor of the solid image sensor element can be modified.

Further, in the embodiment, the diaphragm 2 is disposed on the objectside of the first lens 3 and the first lens 3 is formed in a meniscusshape with its convex surface facing the object side. Thereby, itbecomes possible to effectively utilize the second face of the firstlens 3.

That is, by making the angle of the abaxial light ray acute with respectto a normal line of the second face of the first lens 3 in a directionaway from an optical axis 8, the refractive power (correcting effect) ofthe second face of the first lens 3 can be increased.

Thereby, each aberration (especially, comma aberration and chromaticaberration) generated off the axis can be effectively corrected.

On the contrary, if the shape of the second face of the first lens 3 isprotruded towards the image surface side, or the diaphragm 2 is disposedon the image surface side than the first lens 3, it is not possible toincrease the refractive power of the second face of the first lens 3.Thus, the above-described effect of correcting each aberration generatedoff axis is extremely limited.

It is further effective to form the second face of the first lens 3 tobe aspherical face from a point of view that it increases the effect ofcorrecting each aberration generated off the axis. Furthermore, in thiscase, it is desirable to form the second face of the first lens 3 to bethe aspherical surface whose curvature increases as going away from theoptical axis 8. With this, the angle of the abaxial light ray can bemade more acute with respect to the normal line of the second face ofthe first lens 3 in the direction away from the optical axis 8. Thus,the above-described effect of correcting each aberration generated offthe axis can be more effectively increased.

Further, in the embodiment, the shape of the second face of the secondlens 4 is protruded towards the image surface side. Thus, highertelecentricity can be maintained and the incident angle with respect tothe sensor of the solid image sensor element can be more effectivelycontrolled. In addition, it is preferable to form the second face of thesecond lens 4 to be the aspherical face whose curvature increases asgoing away from the optical axis 8. With this, still highertelecentricity can be maintained and the incident angle with respect tothe sensor of the solid image sensor element can be more effectivelycontrolled.

Furthermore, in the embodiment, the imaging lens system 1 is to satisfyconditions expressed by each of the following expressions (1)-(7);1.25≧L/fl≧0.8   (1)1≧f ₁ /f ₂>0.55   (2)1.8≧f ₁ /fl≧1.0   (3)4.0≧f ₂ /fl≧1.5   (4)1.0≧d ₂ /d ₁≧0.5   (5)0.27≧d ₁ /fl≧0.1   (6)0.27≧d ₃ /fl≧0.1   (7)where, L in the expression (1) is the entire length of the lens system,i.e. the optical distance from the surface which is physically closestto the object side to the image taking surface. More specifically, whenthe first face of the first lens 3 in the vicinity of the optical axis 8is positioned closer to the image surface side than the diaphragm 2, thedistance form the diaphragm 2 to the image taking surface is L. In themeantime, as described above, when the first face of the first lens 3 inthe vicinity of the optical axis 8 is positioned close to the objectside than the diaphragm 2 through the diaphragm 2, the distance from thefirst face of the first lens 3 (not the diaphragm 2) to the image takingsurface is L. Further, fl in the expressions (1), (3), (6) and (7) isthe focal distance of the entire lens system. Furthermore, f₁ in theexpressions (2) and (3) is the focal distance of the first lens 3, andf₂ in the expressions (2) and (4) is the focal distance of the secondlens 4. Moreover, d₁ in the expressions (5) and (6) is the centerthickness of the first lens 3, d₂ in the expression (5) is the distancebetween the first lens 3 and the second lens 4 on the optical axis 8,and d₃ in the expression (7) is the center thickness of the second lens4.

When the value of the L/fl exceeds the value (1.25) shown in theexpression (1), the entire optical system becomes large-scaled againstthe demand for reducing the size and weight. In the meantime, when thevalue of L becomes below the value (0.8) shown in the expression (1),the entire optical system is down-sized. Thus, the productivity isdeteriorated and it becomes difficult to maintain the opticalperformance.

Accordingly, with the embodiment, it becomes possible to sufficientlyreduce the size and weight of the entire optical system whilemaintaining the necessary back focus distance through setting the valueof L/fl to satisfy the expression (1). Furthermore, it enables tomaintain an excellent optical performance and to improve theproductivity as well.

It is more preferable that the relationship between L and fl satisfy anexpression 1.2≧L/fl≧1.1.

Further, when the value of f₁/f₂ exceeds the value (1.0) shown in theexpression (2), the power of the second lens 4 becomes too strong sothat the productivity is deteriorated. In addition, the back focusbecomes too long so that reduction of the size and weight becomesdifficult. In the meantime, when the value of f₁/f₂ becomes below thevalue (0.55) shown in the expression (2), the productivity of the firstlens 3 is deteriorated and it becomes difficult to maintain thenecessary back focus distance.

Accordingly, with the embodiment, it enables to further improve theproductivity by setting the value of f₁/f₂ to satisfy the expression(2). In addition, it enables to further reduce the size and weight ofthe entire optical system while more effectively maintaining thenecessary back focus distance.

It is more preferable that the relationship between f₁ and f₂ satisfy anexpression 1.0≧f₁/f₂≧0.6.

Furthermore, when the value of f₁/fl exceeds the value (1.8) shown inthe expression (3), the back focus distance becomes too long so thatreduction of the size and weight becomes difficult. In the meantime,when the value of f₁/fl becomes below the value (1.0) shown in theexpression (3), the productivity of the first lens is deteriorated.

Accordingly, with the embodiment, it becomes possible to further reducethe size and weight and to improve the productivity by setting the valueof f₁/fl to satisfy the expression (3).

It is more preferable that the relation between f₁ and fl satisfy anexpression 1.7≧f₁/fl≧1.3.

Moreover, when the value of f₂/fl exceeds the value (4.0) shown in theexpression (4), the productivity of the first lens 3 is deteriorated. Inaddition, it becomes difficult to maintain the necessary back focusdistance. In the meantime, when the value of f₂/fl becomes below thevalue (1.5) shown in the expression (4), the power of the second lens 4becomes too strong so that the productivity is deteriorated.

Accordingly, with the embodiment, it enables to improve the productivitywhile further maintaining the necessary back focus distance to beappropriate by setting the value of f₂/fl to satisfy the expression (4).

It is more preferable that the relation between f₂ and fl satisfy anexpression 2.4≧f₂/fl≧1.5.

Further, when the value of d₂/d₁ exceeds the value (1.0) shown in theexpression (5), it becomes necessary to increase the power of the firstlens 3 and the second lens 4. Thus, it becomes difficult to manufactureeach of the lenses 3 and 4. Also, the height of light rays passingthrough the surface of the second lens 4 on the image surface sidebecomes high. Therefore, the power of the aspherical face is increasedso that manufacture of the lenses becomes more difficult. In themeantime, when the value of d₂/d₁ becomes below the value (0.5) shown inthe expression (5), the center thickness of the first lens 3 isrelatively thickened. Therefore, it becomes difficult to maintain theback focus distance and also to insert a diaphragm which effectivelylimits the light amount.

Accordingly, with the embodiment, it enables to further improve theproductivity by setting the value of d₂/d₁ to satisfy the expression(5). In addition, it becomes possible to further maintain the necessaryback focus distance to be appropriate and also to maintain the opticalperformance to be more excellent.

It is more preferable that the relation between d₂ and d₁ satisfy anexpression 0.9≧d₂/d₁≧0.5.

Furthermore, when the value of d₁/fl exceeds the value (0.27) shown inthe expression (6), the entire length of the optical system becomes toolong. Thus, reduction of the size and weight becomes difficult. In themeantime, when the value of d₁/fl becomes below the value (0.1) shown inthe expression (6), manufacture of the first lens 3 becomes difficult.

Accordingly, with the embodiment, it enables to further reduce the sizeand weight and to improve the productivity by setting the value of d₁/flto satisfy the expression (6).

It is more preferable that the relation between d, and fl satisfy anexpression 0.25≧d₁/fl≧0.15.

Moreover, when the value of d₃/fl exceeds the value (0.27) shown in theexpression (7), the entire length of the optical system becomes toolong. Thus, reduction of the size and weight becomes difficult. In themeantime, when the value of d₃/fl becomes below the value (0.1) shown inthe expression (6), manufacture of the second lens 4 becomes difficult.

Accordingly, with the embodiment, it enables to further reduce the sizeand weight of the entire optical system by setting the value of d₃/fl tosatisfy the expression (7). In addition, it enables to further improvethe productivity.

It is more preferable that the relation between d₃ and fl satisfy anexpression 0.25≧d₃/fl≧0.15.

In addition to the above-described structure, it is further desirable toform the second lens 4 as a meniscus lens.

With this, it becomes possible to improve the optical performance of theperiphery without imposing a burden on the shapes of the first lens 3and the second lens 4. Also, it enables to more effectively utilize thelight rays which make incidence to the periphery of the solid imagesensor element.

Furthermore, as the first face of the second lens 4, it is alsodesirable to be formed as a convex surface in which a part close to theoptical axis protrudes towards the object side and also to be formed asan aspherical face with an inflection point.

The inflection point of the first face of the second lens 4 is, on across section of the second lens 4 which is cut at a cross sectionincluding the optical axis 8, a point at which a tangent in touch with acurve (a curve on the cross section) of the first face of the secondlens 4 changes a sign of the slope thereof.

Thus, as described above, in the case of the convex surface where thecenter part of the first face of the second lens 4 is facing the objectside, the peripheral part surrounding the center part of the first faceis to change its surface shape as a concave surface towards the objectside at the inflection point as a boundary.

With this, it enables to further improve the optical performance of theperiphery without further imposing a burden on each shape of the lenses3 and 4, so that the light rays passing through each of the lenses 3 and4 can be more effectively utilized.

The first face of the second lens 4 may be formed to have a plurality ofinflection points in order from the optical axis 8 towards the peripheryside. In this case, it enables to correct each aberration moreexcellently.

Furthermore, in addition to the above-described structure, the outer endpart of the effective diameter of the first face of the second lens 4 isdesirable to be positioned at a point closer to the object side than apoint on the optical axis 8 of the first face of the second lens.

Thereby, the optical performance in the periphery can be more improved.Further, it is not only advantageous at the time handling the lens butalso advantageous at the time of assembling for making a unit bymounting the lens to a barrel.

In addition to the above-described structure, the diaphragm 2 isdesirable to satisfy the condition expressed by the following expression(8).

S in the expression (8) is the distance between the diaphragm 2 on theoptical axis 8 and the optical surface closest to the object side, i.e.the distance between the diaphragm 2 on the optical axis 8 and the firstface of the first lens 3. Also, S is a physical distance, and thediaphragm 2 may be either on the object side or the image surface sidewith respect to the point on the optical axis 8 of the first face of thefirst lens 3.0.2≧S   (8)

With this, in addition, the telecentricity can be more effectivelymaintained and more reduction of the size and weight can be achieved.

S is more preferable to satisfy an expression 0.15≧S.

Furthermore, in addition to the above-described structure, it isdesirable to satisfy the condition expressed by the following expression(9).

Bfl in the expression (9) is the back focus distance, i.e. the distance(air reduced length) on the optical axis 8 from the lens end face (thesecond face of the second lens 4) to the image taking surface 7.0.8≧Bfl/fl≧0.4   (9)

With this, reduction of the size and weight can be more effectivelyachieved. Also, productivity is more improved and it can be more easilyhandled at the time of assembling.

It is more preferable that the relationship between Bfl and fl satisfyan expression 0.7≧Bfl/fl≧0.5.

Furthermore, in addition to the above-described structure, it isdesirable to satisfy the condition expressed by the following expression(10).2.5≧Bfl≧1.2   (10)

With this, reduction of the size and weight can be more effectivelyachieved. Also, productivity is more improved and it can be more easilyhandled at the time of assembling.

Bfl is more preferable to satisfy an expression 2.0≧Bfl≧1.3.

Furthermore, in addition to the above-described structure, it isdesirable to satisfy the condition expressed by the following expression(11).

In the expression (11), r₄ is the radius curvature of the second face ofthe second lens 4.−0.5≧r ₄ /fl≧−6.0   (11)

With this, processing of the optical surface can be more easilyperformed and each aberration in the periphery can be more excellentlycorrected.

It is more preferable that the relationship between r₄ and fl satisfy anexpression −0.7≧r₄/fl≧−1.2.

Further, in addition to the above-described structure, fl is desirableto satisfy an expression 5.0≧fl≧2.0 (more preferably, 3.5≧fl≧2.0).

Thereby, it becomes a structure which is more preferable as a lens for acamera module to be mounted on a portable terminal and the like.

Furthermore, a resin material for molding the first lens 3 and thesecond lens 4 may have any composition as long as it is a material withtransparency to be used for molding optical components, e.g., acryl,polycarbonate, amorphous polyolefin resin, etc. However, in order tofurther improve the manufacturing efficiency and more reduction ofmanufacturing cost, it is desirable to use the same resin material forboth lenses 3 and 4.

EXAMPLES

Next, EXAMPLES of the present invention will be described by referringto FIG. 2 -FIG. 11.

In this embodiment, F No denotes F number and r denotes the curvatureradius of the optical surface (the center radius curvature in the caseof a lens). Further, d denotes the distance to the next optical surface.Furthermore, nd denotes the index of refraction when the d line (yellow)is irradiated, and υ d denotes the Abbe number also when the d line isirradiated.

k, A, B, C and D denote each coefficient in a following expression (12).That is, the shape of the aspherical surface is expressed by thefollowing expression provided that the direction of the optical axis 8is taken as the Z axis, the direction orthogonal to the optical axis 8is the X axis, the traveling direction of light is positive, k is theconstant of cone, A, B, C, D are the aspherical coefficients, and r isthe curvature radius.Z(X)=r ⁻¹ X ²/[1+{1−(k+1)r ⁻² X ² } ^(1/2) ]+AX ⁴ +BX ⁶ +CX ⁸ +DX ¹⁰  (12)

First Example

FIG. 2 shows FIRST EXAMPLE of the present invention. In FIRST EXAMPLE,like the imaging lens system 1 shown in FIG. 1, a diaphragm 2 wasdisposed on the object side of the first face of the first lens 3, and acover glass as a filter 6 is disposed between the second face of thesecond lens 4 and an image taking surface 7. The first face of the firstlens 3 is disposed at a position closer to the object side than thediaphragm 2 through the diaphragm 2.

The imaging lens system 1 of FIRST EXAMPLE was set under the followingcondition.

Lens Data

L=2.92 mm, fl=4.09 mm, f₁=4.09 mm, f₂=4.37 mm, d₁=0.50 mm, d₂=0.30 mm,d₃=0.55 mm, r₄=−2.564 mm, F no=2.8 Face Number r d nd νd (Object Point)1 (First Face of First Lens) 0.769 0.500 1.525 56.0 2 (Second Face ofFirst Lens) 0.930 0.300 3 (First Face of Second Lens) 20.000 0.550 1.52556.0 4 (Second Face of Second Lens) −2.564 0.000 5 (First Face of CoverGlass) 0.000 0.300 1.516 64.0 6 (Second Face of Cover Glass) 0.000(Image Surface)

Diadhragm 2 was disposed at a position 0.1 mm towards the image surfaceside from the point on the optical axis 8 of the first face of the firstlens 3. Face Number k A B C D 1 −3.77E−2 4.70E−2 −2.40E−1 1.60E −2.20E 2−1.00E 4.16E−1 3.65E−1 4.30E 0 3 1.17E+2 −2.06E−1 −4.88E−2 −2.00E 0 48.29E 7.15E−2 −3.80E−1 5.74E−1 −6.93E−1

Under such condition, L/fl=1.13 was achieved, thereby satisfying theexpression (1), and f₁/f₂=0.94 was achieved, thereby satisfying theexpression (2). Further, f₁/fl=1.59 was achieved, thereby satisfying theexpression (3), and F₂/fl=1.69 was achieved, thereby satisfying theexpression (4). Furthermore, d₂/d₁=0.60 was achieved, thereby satisfyingthe expression (5), and d₁/fl=0.19 was achieved, thereby satisfying theexpression (6). Also, d₃/fl=0.21 was achieved, thereby satisfying theexpression (7). Further, S=0.10 mm was achieved, thereby satisfying theexpression (8), and Bfl/fl=0.61 was achieved, thereby satisfying theexpression (9). Moreover, Bfl=1.57 mm was achieved, thereby satisfyingthe expression (10), and r₄/fl=−0.63 was achieved, thereby satisfyingthe expression (11).

FIG. 3 shows the spherical aberration, astigmatism, and distortion ofthe imaging lens system 1 of FIRST EXAMPLE.

Each of the spherical aberration, astigmatism, and distortion was almostsatisfied. According to the result, it can be seen that a sufficientoptical property can be obtained.

Second Example

FIG. 4 shows SECOND EXAMPLE of the present invention. In SECOND EXAMPLE,like the imaging lens system 1 shown in FIG. 1, a diaphragm 2 wasdisposed on the object side of the first face of the first lens 3, and acover glass as a filter 6 is disposed between the second face of thesecond lens 4 and the image taking surface 7.

The imaging lens system 1 of SECOND EXAMPLE was set under the followingcondition.

Lens Data

L=3.60 mm, fl=3.11 mm, f₁=4.39 mm, f₂=6.82 mm, d₁=0.75 mm, d₂=0.403 mm,d₃=0.75 mm, r₄=−15.152 mm, F no=3.2 Face Number r d nd νd (Object Point)1 (First Face of First Lens) 1.055 0.750 1.531 56.0 2 (Second Face ofSecond Lens) 1.453 0.403 3 (First Face of Second Lens) 4.675 0.750 1.53156.0 4 (Second Face of Second Lens) −15.152 0.000 5 (First Face of CoverGlass) 0.000 0.300 1.516 64.0 6 (Second Face of Cover Glass) 0.000(Image Surface)

Diaphragm 2 was disposed at a position 0.1 mm towards the object sidefrom the point on the optical axis 8 of the first face of the first lens3. Face Number k A B C D 1 −2.17E−1 1.56E−2 −1.00E−1 5.47E−1 −5.26E−1 22.45E −6.65E−2 −6.95E−2 −1.38E−1 8.61E−1 3 0 −7.91E−2 −5.26E−1 1.06E−1.67E 4 −1.20E+4 −4.25E−2 5.83E−3 −8.12E−2 1.71E−2

Under such condition, L/fl=1.16 was achieved, thereby satisfying theexpression (1), and f₁/f₂=0.64 was achieved, thereby satisfying theexpression (2). Further, f₁/fl=1.41 was achieved, thereby satisfying theexpression (3), and f₂/fl=2.19 was achieved, thereby satisfying theexpression (4). Furthermore, d₂/d₁=0.54 was achieved, thereby satisfyingthe expression (5), and d₁/fl=0.24 was achieved, thereby satisfying theexpression (6). Also, d₃/fl=0.24 was achieved, thereby satisfying theexpression (7). Further, S=0.0 mm was achieved, thereby satisfying theexpression (8), and Bfl/fl=0.51 was achieved, thereby satisfying theexpression (9). Moreover, Bfl=1.6 mm was achieved, thereby satisfyingthe expression (10), and r₄/fl=−4.872 was achieved, thereby satisfyingthe expression (11).

FIG. 5 shows the spherical aberration, astigmatism, and distortion ofthe imaging lens system 1 of SECOND EXAMPLE.

Each of the spherical aberration, astigmatism, and distortion was almostsatisfied. According to the result, it can be seen that a sufficientoptical property can be obtained.

Third Example

FIG. 6 shows THIRD EXAMPLE of the present invention. In THIRD EXAMPLE,like the imaging lens system 1 shown in FIG. 1, a diaphragm 2 wasdisposed on the object side of the first face of the first lens 3, and acover glass as a filter 6 is disposed between the second face of thesecond lens 4 and the image taking surface 7. The first face of thefirst lens 3 is disposed at a position closer to the object side thanthe diaphragm 2 through the diaphragm 2.

The imaging lens system 1 of THIRD EXAMPLE was set under the followingcondition.

Lens Data

L=2.88 mm, fl=2.53 mm, f₁=3.50 mm, f₂=5.21 mm, d₁=0.5 mm, d₂=0.35 m,d₃=0.55 mm, r₄=−2.60 mm, F no=2.8 Face Number r d nd νd (Object Point) 1(First Face of First Lens) 0.800 0.500 1.531 56.0 2 (Second Face ofFirst Lens) 1.100 0.350 3 (First Face of Second Lens) −40.000 0.5501.531 56.0 4 (Second Face of Second Lens) −2.600 0.000 5 (First Face ofCover Glass) 0.000 0.300 1.516 64.0 6 (Second Face of Cover Glass) 0.000(Image Surface)

Diaphragm 2 was disposed at a position 0.1 mm towards the image surfaceside from the point on the optical axis 8 of the first face of the firstlens 3. Face Number k A B C D 1 −1.78E−1 9.23E−3 8.98E−1 −4.15E 9.03E 22.86E 4.36E−2 −7.07E−1 2.59E −1.11E 3 −2.44E+5 −2.37E−1 8.13E−2 −2.15E 04 −9.15E+1 −2.99E−1 3.02E−1 −4.45E−1 −3.77E−2

Under such condition, L/fl=1.14 was achieved, thereby satisfying theexpression (1), and f₁/f₂=0.67 was achieved, thereby satisfying theexpression (2). Further, f₁/fl=1.38 was achieved, thereby satisfying theexpression (3), and f₂/fl=2.06 was achieved, thereby satisfying theexpression (4). Furthermore, d₂/d₁=0.70 was achieved, thereby satisfyingthe expression (5), and d₁/fl=0.20 was achieved, thereby satisfying theexpression (6). Also, d₃/fl=0.22 was achieved, thereby satisfying theexpression (7). Further, S=0.10 mm was achieved, thereby satisfying theexpression (8), and Bfl/fl=0.58 was achieved, thereby satisfying theexpression (9). Moreover, Bfl=1.48 mm was achieved, thereby satisfyingthe expression (10), and r₄/fl=−1.03 was achieved, thereby satisfyingthe expression (11).

FIG. 7 shows the spherical aberration, astigmatism, and distortion ofthe imaging lens system 1 of THIRD EXAMPLE.

Each of the spherical aberration, astigmatism, and distortion was almostsatisfied. According to the result, it can be seen that a sufficientoptical property can be obtained.

Fourth Example

FIG. 8 shows FOURTH EXAMPLE of the present invention. In FOURTH EXAMPLE,like the imaging lens system 1 shown in FIG. 1, a diaphragm 2 wasdisposed on the object side of the first face of the first lens 3, and acover glass as a filter 6 is disposed between the second face of thesecond lens 4 and the image taking surface 7.

The imaging lens system 1 of FOURTH EXAMPLE was set under the followingcondition.

Lens Data

L=mm, fl=3.17 mm, f₁=4.71 mm, f₂=5.72 mm, d₁=0.65 mm, d₂0.45 mm, d₃=0.65mm, r₄=−2.667 mm, F no=3.2 Face Number r d nd νd (Object Point) 1 (FirstFace of First Lens) 1.053 0.650 1.531 56.0 2 (Second Face of SecondLens) 1.429 0.450 3 (First Face of Second Lens) −20.000 0.650 1.531 56.04 (Second Face of Second Lens) −2.667 0.000 5 (First Face of CoverGlass) 0.000 0.300 1.516 64.0 6 (Second Face of Cover Glass) 0.000(Image Surface)

Diaphragm 2 was disposed at the same position as the point on theoptical the first face of the first lens 3. Face Number k A B C D 1−1.48E−1 3.77E−2 −2.08E−1 7.68E−1 −1.58E−1 2 3.77E −3.21E−2 −5.45E−17.24E−1 −2.15E−1 3 −2.44E+5 −2.25E−1 1.39E−1 −8.12E−1 0 4 −1.07E+2−3.36E−1 3.80E−1 −4.02E−1 8.88E−2

Under such condition, L/fl=1.19 was achieved, thereby satisfying theexpression (1), and f₁/f₂=0.82 was achieved, thereby satisfying theexpression (2). Further, f₁/fl=1.49 was achieved, thereby satisfying theexpression (3), and f₂/fl=1.80 was achieved, thereby satisfying theexpression (4). Furthermore, d₂/d₁=0.69 was achieved, thereby satisfyingthe expression (5), and d₁/fl=0.21 was achieved, thereby satisfying theexpression (6). Also, d₃/fl=0.21 was achieved, thereby satisfying theexpression (7). Further, S=0.0 mm was achieved, thereby satisfying theexpression (8), and Bfl/fl=0.61 was achieved, thereby satisfying theexpression (9). Moreover, Bfl=1.93 mm was achieved, thereby satisfyingthe expression (10), and r₄/fl=−0.841 was achieved, thereby satisfyingthe expression (11).

FIG. 9 shows the spherical aberration, astigmatism, and distortion ofthe imaging lens system 1 of FOURTH EXAMPLE.

Each of the spherical aberration, astigmatism, and distortion was almostsatisfied. According to the result, it can be seen that a sufficientoptical property can be obtained. FIFTH EXAMPLE

FIG. 10 shows FIFTH EXAMPLE of the present invention. In FIFTH EXAMPLE,like the imaging lens system 1 shown in FIG. 1, a diaphragm 2 wasdisposed on the object side of the first face of the first lens 3, and acover glass as a filter 6 is disposed between the second face of thesecond lens 4 and the image taking surface 7.

The imaging lens system 1 of FIFTH EXAMPLE was set under the followingcondition.

Lens Data

L=3.16 mm, fl=2.78 mm, f₁=3.85 mm, f₂=5.73 mm, d₁=0.55 mm, d₂=0.38 mm,d₃=0.60 mm, r₄=−2.860 mm, F no =2.8 Face Number r d nd νd (Object Point)1 (First Face of First Lens) 0.880 0.550 1.531 56.0 2 (Second Face ofFirst Lens) 1.210 0.380 3 (First Face of Second Lens) −44.000 0.6001.531 56.0 4 (Second Face of Second Lens) −2.860 0.000 5 (First Face ofCover Glass) 0.000 0.300 1.516 64.0 6 (Second Face of Cover Glass) 0.000(Image Surface)

Diaphragm 2 was disposed at a position 0.1 mm towards the image surfacethe point on the optical axis 8 of the first face of the first lens 3.Face Number k A B C D 1 −1.48E−1 6.93E−3 5.57E−1 −2.13E 3.83E 2 2.86E3.27E−2 −4.39E−1 1.33E −4.69E−1 3 −2.44E+5 −1.78E−1 5.05E−2 −1.10E 0 4−9.15E+1 −2.25E−1 1.88E−1 −2.28E−1 −1.60E−2

Under such condition, L/fl =1.14 was achieved, thereby satisfying theexpression (1), and f₁/f₂=0.67 was achieved, thereby satisfying theexpression (2). Further, f₁/fl=1.38 was achieved, thereby satisfying theexpression (3), and f₂/fl=2.06 was achieved, thereby satisfying theexpression (4). Furthermore, d₂/d₁=0.69 was achieved, thereby satisfyingthe expression (5), and d₁/fl=0.20 was achieved, thereby satisfying theexpression (6). Also, d₃/fl=0.22 was achieved, thereby satisfying theexpression (7). Further, S=0.10 mm was achieved, thereby satisfying theexpression (8), and Bfl/fl=0.59 was achieved, thereby satisfying theexpression (9). Moreover, Bfl=1.63 mm was achieved, thereby satisfyingthe expression (10), and r₄/fl=−1.028 was achieved, thereby satisfyingthe expression (11).

FIG. 11 shows the spherical aberration, astigmatism, and distortion ofthe imaging lens system 1 of FIFTH EXAMPLE.

Each of the spherical aberration, astigmatism, and distortion was almostsatisfied. According to the result, it can be seen that a sufficientoptical property can be obtained.

The present invention is not limited to the above-described embodimentand EXAMPLES but various modifications are possible as necessary.

For example, a light amount restriction plate may be provided betweenthe second face of the first lens 3 and the first face of the secondlens 4 as necessary.

1. An imaging lens system used for forming an image of an object on animage taking surface of a solid image sensor element, comprising: inorder from an object side towards an image surface side, a diaphragm, afirst lens which is a meniscus lens having positive power whose convexsurface facing said object side, and a second lens which is a lenshaving a positive power whose convex surface facing said image surfaceside, wherein conditions expressed by each of following expressions(1)-(7) are to be satisfied;1.25≧L/fl≧0.8   (1)1≧f ₁ /f ₂≧0.55   (2)1.8≧f₁ /fl≧1   (3)4≧f ₂ /fl≧1.5   (4)1≧d ₂ /d ₁≧0.5   (5)0.27≧d ₁ /fl ≧0.1   (6)0.27≧d ₃ /fl≧0.1   (7) where, L: entire length of said lens system fl:focal distance of entire lens system f₁: focal distance of said firstlens f₂: focal distance of said second lens d₁: center thickness of saidfirst lens d₂: distance between said first lens and second lens on anoptical axis d₃: center thickness of said second lens
 2. The imaginglens system according to claim 1, wherein said second lens is formed asa meniscus lens.
 3. The imaging lens system according to claim 1,wherein an object-side surface of said second lens is protruded towardssaid object side in an area near an optical axis and is also formed asan aspherical face with an inflection point.
 4. The imaging lens systemaccording to claim 3, wherein an outer end part of effective diameter inan object-side surface of said second lens is positioned closer to saidobject side than a point on the optical axis on said object-side surfaceof said second lens.
 5. The imaging lens system according to any one ofclaims 1-4, wherein, further, said diaphragm satisfies a conditionexpressed by a following expression (8);0.2≧S   (8) where, S: distance between said diaphragm on the opticalaxis and an optical surface closest to said object side
 6. The imaginglens system according to claim 5, wherein, further, condition expressedby a following expression (9) is to be satisfied;0.8≧Bfl/fl≧0.4   (9) where, Bfl: back focus distance (distance from alens end surface to said image taking surface on said optical axis (airreduced length))
 7. The imaging lens system according to claim 6,wherein, further, condition expressed by a following expression (10) isto be satisfied;2.5≧Bfl≧1.2   (10)
 8. The imaging lens system according to claim 7,wherein, further, condition expressed by a following expression (11) isto be satisfied;−0.5≧r ₄ /fl≧−6.0   (11) where, r₄: curvature radius of a surface ofsaid second lens on said image surface side